class BankAccount:
    def __init__(self, owner, balance=0):
        if balance < 0: raise ValueError("negative opening balance")
        self.owner = owner
        self._balance = balance
    @property
    def balance(self): return self._balance
    def deposit(self, amount):
        if amount <= 0: raise ValueError("deposit must be positive")
        self._balance += amount
    def withdraw(self, amount):
        if amount <= 0 or amount > self._balance: raise ValueError("invalid withdrawal")
        self._balance -= amount

account = BankAccount("Sam", 50)
account.deposit(25)
account.withdraw(10)
assert account.balance == 65
print(account.balance)